1. Field of the Invention
This invention relates to a photo-mask used in the lithography process of the manufacture of semiconductive elements or liquid crystal display elements, and further to a method of exposing and a projection-exposing apparatus for transferring the pattern of a mask onto a photosensitive substrate.
2. Related Background Art
A method of transferring a pattern formed on a mask or a reticle (hereinafter referred to as the reticle) onto a substrate is adopted in the lithography process for forming the circuit pattern of a semiconductive element or the like. Photosensitive photoresist is applied onto the substrate (semiconductive wafer or glass plate), and the circuit pattern is transferred to photosensitive thin film (photoresist) in conformity with an applied optical image, i.e., the pattern shape of the transparent portion of the reticle pattern. In a projection-exposing apparatus, the image of the reticle pattern is projected onto the wafer through a projection optical system.
The projection-exposing apparatus is divided broadly into the collective exposure type and the scanning exposure type. Projection-exposing apparatuses of the collective exposure type include projection-exposing apparatuses of the step and repeat type, i.e., so-called steppers. Also, projection-exposing apparatuses of the scanning exposure type include mirror projection aligners in which a reticle and a wafer are opposed to each other with a projection optical system interposed therebetween and relative scanning is effected to thereby effect exposure while the reticle is illuminated by arcuate slit illuminating light. Further, recently, in the scanning exposure type, a new system for achieving high resolving power has been proposed as a step and scan type in pages 424-433 of SPIE, Vol. 1088, Optical Laser Microlithography II (1989). The step and scan type is the use of a scan type in which a reticle is one-dimensionally scanned and at the same time, a wafer is one-dimensionally scanned at a speed synchronized therewith, with a type in which the wafer is step-moved in a direction orthogonal to the scanning exposure direction.
Now, the projection optical system used in the exposing apparatus of the scanning exposure type comprises chiefly a reflecting element alone, and uses arcuate slit illuminating light. This is for obtaining the advantage that various aberrations become substantially zero in the narrow range (zonal) of an image height point spaced apart by a predetermined distance from the optical axis of the projection optical system. Further, the projection optical system comprising a reflecting element (a reflection projection system), as compared with a projection optical system comprising a refracting element (a refraction projection system), has the advantage that it can use illuminating light of a wider wavelength range as exposure light. This means the obtainment of the effect of reducing a standing wave created within photoresist on the wafer.
Also, the reflection projection system, unlike the refraction projection system, does not have the necessity of taking the light transmittance characteristic of the optical element used into account. Accordingly, in the reflection projection system, even an exposing apparatus using energy rays of the vacuum ultraviolet range which, in the refraction projection system, is difficult to realize because of transmittance can be realized. However, the good image range of the projection optical system is limited to an arcuate area and therefore, to secure a wider exposure area, scanning during exposure is necessary. An example of the projection optical system, particularly the reflection refraction reduction projection system, suitable for use in the step and scan system is disclosed, for example, in U.S. Pat. No. 4,747,678. Also, besides the step and scan system using arcuate slit illuminating light, an attempt to apply a projection optical system having a circular image field (the full field type) to the step and scan system is proposed, for example, in U.S. application Ser. No. 253,717 (Oct. 5, 1989).
On the other hand, a projection-exposing apparatus of the collective exposure type (for example, a stepper) has adopted a construction in which in the plane of an illuminating optical system (hereinafter referred to as the pupil plane of the illuminating optical system) which becomes the Fourier transformation plane of that surface of a reticle on which a pattern exists, or in a plane near it, an illuminating light beam is limited to a substantially circular shape (or a rectangular shape) centering at the optical axis of the illuminating optical system and illuminates the reticle. Therefore, the illuminating light beam has been incident on the reticle at an angle approximate to perpendicularity thereto. Also, on the reticle (a glass substrate such as quartz) used in this apparatus, there has been depicted a circuit pattern comprised of a transmitting portion (the naked surface portion of the substrate) of which the transmittance to the illuminating light beam is approximately 100% and a light-intercepting portion (chromium) of which the transmittance is approximately 0%. In the apparatus of this kind, the illuminating light beam applied to the reticle is diffracted by the reticle pattern, and 0-order diffracted light and .+-.1st-order diffracted lights are created from the pattern. These diffracted lights are condensed by the projection optical system and interference fringes, i.e., the image of the reticle pattern, is formed on a wafer. At this time, the angle .theta. (the reticle side) formed between the 0-order diffracted light and the .+-.1st-order diffracted lights is determined by sin .theta.=.lambda./P when the wavelength of exposure light is .lambda. (.mu.m) and the reticle side numerical aperture of the projection optical system is NA.
Now, if the pattern pitch becomes minute, sin .theta. will become great, and if sin .theta. becomes greater than the reticle side numerical aperture (NA) of the projection optical system, the .+-.1st-order diffracted lights will be limited by the effective diameter of the plane of the projection optical system which is the Fourier transformation plane of the reticle pattern (hereinafter referred to as the pupil plane of the projection optical system) and will become unable to be transmitted through the projection optical system. That is, only the 0-order diffracted light will arrive at the wafer and interference fringes (the image of the pattern) will not be created. Accordingly, when in the above-described apparatus, use is made of a reticle comprising only the aforedescribed transmitting portion and light-intercepting portion (hereinafter referred to as the usual reticle), the degree of minuteness (the maximum pattern pitch) P of the reticle pattern which can be resolved on the wafer is given by the relational expression that P.congruent..lambda./NA, from sin .theta.=NA.
From this, the minimum pattern size is a half of the pitch P and therefore, the minimum pattern size is of the order of 0.5.times..lambda./NA. In the actual photolithography process, however, some degree of depth of focus becomes necessary due to the influences of the curvature of the wafer, the level difference of the wafer by the process, etc. or the thickness of photoresist itself. Therefore, the practical minimum resolution pattern size is expressed as k.times..lambda./NA, where k is called a process coefficient and usually is of the order of 0.6-0.8.
Thus, to expose and transfer a pattern more minute than at present in the prior-art apparatus, it has been necessary to use a light source of short wavelength for exposure (such as an excimer laser source) or to use a projection optical system of great numerical aperture. However, making the wavelength of the exposure light source shorter than at present is difficult at the present point of time because the running cost of the excimer laser source or the like increases and the development of resist is difficult since in the short wavelength range, the absorption of light becomes great. Also, the numerical aperture of the projection optical system is already approximate to the theoretic limit even at present and it is nearly desperate to make the numerical aperture greater than that. Even if it is possible to make the numerical aperture greater than at present, the apparatus will become bulky and costly and moreover, the depth of focus determined by .+-..lambda./NA.sup.2 will sharply decrease with an increase in the numerical aperture and therefore, the depth of focus necessary for practical use will become smaller, and this leads to the problem that a practical exposing apparatus cannot be provided.
Therefore, it has also been proposed to use a phase shift reticle provided with a phase shifter (such as dielectric thin film) for shifting the phase of transmitted light from particular one of the transmitting portions of the circuit pattern of the reticle by .pi. (rad) relative to transmitted light from the other transmitting portion. The phase shift reticle is disclosed, for example, in Japanese Patent Publication No. 62-50811, and if this phase shift reticle is used, the transfer of a pattern more minute than when the usual reticle is used becomes possible. That is, there is the effect of improving the resolving power.
If a phase shift reticle of this kind is used, the transfer of a pattern which is minute as compared with the usual reticle is possible, but the phase shift reticle is difficult to manufacture and moreover, the manufacturing process becomes complex and correspondingly, costs become higher. For example, the usual reticle can be provided simply by forming a light intercepting pattern by chromium or the like on a glass substrate, whereas the phase shift reticle requires the formation of a phase shifter pattern, discretely from the formation of a light intercepting pattern. Accordingly, in the manufacture, at least two patternings and the alignment of those patterns are necessary. Moreover, a method of inspecting the defect of the phase shift reticle has not yet been established and many other problems are left still to be solved, and it is difficult at present to put the phase shift reticle into practical use.
So, attempts have recently been made to make the transfer of a minute pattern possible by the optimization of the illuminating condition or the contrivance of the exposing method. For example, there has been proposed a method of selecting a combination of the optimum numerical aperture (i.e., coherence factor .sigma.) of an illuminating optical system and the numerical aperture (N.A.) of a projection optical system relative to a pattern of a particular line width for each line width of the pattern to thereby improve the resolution and the depth of focus. Further, there has been proposed a zonal illuminating method of prescribing the light quantity distribution of an illuminating light beam in the pupil plane of the illuminating optical system or a plane near it into a zonal shape, or the inclined illuminating method (the deformed light source method) of limiting the light quantity distribution of an illuminating light beam to a plurality of discrete partial areas eccentric with respect to the optical axis of the illuminating optical system, inclining the illuminating light beam by a predetermined angle correspondingly to the periodicity of a reticle pattern and applying it. Particularly, the deformed light source method was announced in the autumn meeting of the Applied Physical Society in 1991, and has further been filed as U.S. application Ser. No. 791,138 (Nov. 13, 1991) and U.S. application Ser. No. 847,030 (Apr. 15, 1992).
However, to make the deformed light source method function effectively, it is necessary to make the angle of incidence of the illuminating light onto the reticle, i.e., the numerical aperture of the illuminating optical system, greater than in the prior art. Therefore, in a projection-exposing apparatus a doping the deformed light source method, the ratio of the numerical aperture of the illuminating optical system to the numerical aperture of the projection optical system, i.e., .sigma. value (coherence factor), becomes great and thus, the illuminating optical system becomes bulkier and more complicated than in the prior-art apparatus and the designing and manufacture of the illuminating optical system become difficult. Further, the bulkiness of the apparatus not only leads to the increased cost of the apparatus itself, but also leads to the problem that the enlargement of a clean room which is the environment in which the projection-exposing apparatus is used is required and the running cost is increased. Such problems arise in both of the collective exposure type and the scanning exposure type.
Also, the formation of the deformed light source shape has been realized by disposing a deformed aperture stop in a secondary light source surface (particularly the exit surface of a fly-eye lens). Thus, a considerable quantity of light is intercepted, and this leads to the problem that illumination (illuminating power) is greatly reduced. Further, the deformed stop decreases the number of lens elements effective in the fly-eye lens, and this also leads to the problem that the effect of uniformizing and averaging the illumination peculiar to the fly-eye lens is reduced and the uniformity of the illumination on the reticle surface and the wafer surface is aggravated.